In recent years, identification technology by which an ID number (identification number) is assigned to an individual object to clarify the history of information thereof, which is utilized for production, management, or the like, has attracted attention. In particular, an RFID (radio frequency identification) technique that transmits and receives data by wireless communication has been widely used. A wireless communication system using the RFID technique is a communication system including a wireless communication device (an interrogator) and a data carrier (a transponder) and communicating data wirelessly therebetween. The wireless communication device refers to a device capable of wirelessly transmitting and receiving signals, such as a reader/writer, a cellular phone, or a personal computer. In this specification, the wireless communication device is typically referred to as a reader/writer. In addition, in this specification, the data carrier is generally called as an RF tag, an ID tag, an IC tag, an IC chip, a wireless tag, an electronic tag, or the like, and the data carrier is typically referred to as an RF tag.
Examples of RF tags include passive RF tags which do not have power sources and active RF tags which incorporate power sources. The passive RF tag has a function to convert a wireless signal generated from a reader/writer (a carrier wave or an amplitude modulation wave generated by superimposing a modulation wave on a carrier wave) into an DC voltage by a rectifier circuit which is provided in the RF tag, and circuits in the RF tag operate using the DC voltage. The strength of the carrier wave is generally expressed as power, and power is attenuated in proportion to the squared distance between a radiant point of the power and a measurement point of the power (hereinafter referred to as a communication distance in this specification). In other words, when the distance between the reader/writer and the RF tag is changed, power which is supplied to the RF tag is also changed in accordance with the change in distance. Then, in the case where the communication distance between the reader/writer and the RF tag is extremely short (for example, in the case where both of them are in contact with each other), high power is supplied to the RF tag. When the RF tag is supplied with high power, the RF tag might malfunction because of incorrect demodulation of signals from the reader/writer or deterioration or break of elements provided in the RF tag.
On the other hand, the wireless communication system using the RFID technique has an advantage that data in a plurality of RF tags can be read at the same time by a reader/writer; however, when the data of the plurality of RF tags is read at the same time, a communication distance between each of the plurality of RF tags and the reader/writer might be different. Further, the communication distance varies from hour to hour, in the case where a carton of products provided with an RF tag is passed through a reader/writer by using a forklift. In view of the foregoing uses of such a wireless communication system, the communication distance might be extremely short unintentionally, and as a result, high power might be supplied to an RF tag.
In response to such a problem, provision of a protection circuit which protects elements in an RF tag when high power is applied to the RF tag has been known (for example, see Reference 1).
The protection circuit has a function to control the state of a dedicated path which makes high power escape. When the communication distance is long and received power is low, the dedicated path which makes high power escape is made in an off state so that the operation of the RF tag is not adversely affected. On the other hand, when the communication distance is short and received power is high, the dedicated path which makes high power escape is made in an on state so that high power is not applied to the elements in the RF tag.